1. Technical Field
The present invention relates generally to iron castings and methods for preparing same, and particularly to ductile iron castings comprised of primary carbides dispersed in an ausferritic matrix and methods for preparing the same.
2. Discussion
The advent of highly complex and very expensive mechanized equipment to perform agricultural functions, such as harvesting, has encouraged manufacturers to develop and provide component parts that can withstand the rigors associated with regular use under harsh operating conditions. For example, the tines on a combine-harvester are subjected to a series of stresses, strains, impacts, and abrasions during the course of a normal operational cycle. Therefore, the materials comprising the tines must be able to achieve the desired level of performance characteristics, such as wear resistance and impact strength, in order to satisfy consumer expectations, reduce component repairs and resulting equipment downtime, and reduce the number of warranty claims.
Generally, these tines are comprised of metallic materials such as iron, to which relatively small amounts of other metallic and/or non-metallic materials have been added, in order to enhance the aforementioned mechanical properties. One particular type of iron that has been used is ductile iron.
Ductile iron, also known as nodular iron or spheroidal iron because of the shape of the graphite particles, is noted primarily for its high strength and toughness. Though made from the same basic materials as gray iron (i.e., 2-4 weight % carbon, 1-3 weight % silicon, with the remainder being iron), a small amount of magnesium, or magnesium and trace amounts of cerium, is inoculated during casting to control the shape and distribution of the graphite. Tensile properties range from 50,000 to 120,000 lb/in.sup.2 (345 to 827 MPa) ultimate strength, 25,000 to 90,000 lb/in.sup.2 (172 to 621 MPa) yield strength, and 2 to 20% elongation. Most ductile iron castings are used as cast, but subsequent heat treatment can be beneficial. Annealing, which provides a ferritic structure (i.e., almost pure iron), maximizes toughness at the expense of strength. Normalizing, often followed by tempering, induces a pearlitic structure (i.e., a lamellar aggregate of ferrite (almost pure iron) and cementite (Fe.sub.3 C)), providing intermediate strength and toughness. A martensitic structure (i.e., an interstitial, super saturated solid solution of carbon in iron having a body-centered tetragonal lattice), induced by quenching, usually in oil, provides the highest strength and hardness, but the least toughness. The modulus of elasticity of ductile iron, 22.times.106 to 25.times.106 lb/in.sup.2 (152,000 to 172,000 MPa), is typically greater than that of gray iron, as is its high-temperature oxidation resistance, but its machinability is about the same. Ductile iron castings are widely used in the automotive industry for crankshafts, camshafts, steering knuckles, pinions, gears, and many other components. They are also used for a variety of machinery applications, marine applications, and equipment used in the paper and glass industries.
In the production of ductile iron, it is typically common practice to add a material generally referred to as an inoculant to the ductile iron in order to prevent the formation of primary iron carbides within the casting. Primary iron carbides are very hard, and thus have excellent wear resistance. However, the effects of primary iron carbides in ductile iron castings are normally quite detrimental as they reduce the machinability, ductility and impact properties. The inoculant is typically a granular ferrosilicon material (e.g., 75 weight % silicon and 25 weight % iron). While the ductile iron is still in the liquid state, the inoculant is added in order to provide sites for the carbon to attach to and begin to solidify as pure carbon (i.e., graphite). When there is not sufficient inoculant nuclei in the ductile iron, the carbon does not have a site to begin to grow and solidifies as an iron-carbon compound called cementite (Fe.sub.3 C), or iron carbide.
Although ductile iron was an improvement over previous materials, it lacked the requisite wear resistance and impact properties that were needed by manufacturers. Other approaches to solve this problem were tried, such as the use of silicon carbide impregnated with aluminum, forgings, die cast materials containing wear resistant coatings, as well as different grades of iron. However, none of these approaches produced a satisfactory solution.
One recent approach has been the use of austempered ductile iron (hereinafter referred to as "ADI"). ADI, an alloyed ductile iron having a structure of ferrite and carbon-rich austenite, has been known for many years but seldomly used because of the difficulty required to induce this structure by heat treatment. Because of the exceptional strength and toughness possible with careful control of heat treatment, however, it has recently emerged as a promising material, especially for automotive and truck applications. The alloying elements are nickel, copper, or molybdenum, or combinations of these, and their purpose is to increase hardenability. The elements delay pearlite formation, permitting the casting to be cooled from austenitizing temperatures to the austempering transformation range without forming pearlite or other high-temperature transformation products during quenching.
Heat treatment involves (1) heating to austenitizing temperature (i.e., generally 1550 to 1700.degree. F. (801 to 912.degree. C.) depending upon the iron chemistry) and holding at this temperature until the structure has transformed to face-centered-cubic austenite and this austenite is saturated with carbon; (2) quenching to a temperature above the martensite start temperature (i.e., 450 to 750.degree. F. (232 to 399.degree. C.) depending upon the iron chemistry) usually in molten salt or a medium capable of providing a quenching rate sufficient to avoid pearlite formation; (3) holding at this temperature for sufficient time austenite (e.g., 30 minutes to 5 hours, depending upon the required properties) to transform the austenite to a structure of acicular ferrite and carbon-rich austenite (i.e., austempering); and (4) cooling to room temperature. No subsequent tempering is necessary. The resulting acicular ferrite and carbon-rich austenite composition is commonly referred to as "ausferrite" connoting its two primary constituents (i.e., austenite and ferrite).
The bainitic reaction temperature, commonly called the austempering temperature, determines mechanical properties. High austempering temperatures promote ductility, fatigue strength, and impact strength, but reduce hardness. Low austempering temperatures increase strength and hardness. Tensile yield strength can range from 80,000 to 180,000 lb/in.sup.2 (552 to 1,240 MPa), with corresponding ultimate strengths of 125,000 to 230,000 lb/in.sup.2 (860 to 1,585 MPa), elongations of up to 10%, and hardness from the range of Brinell 269 to 321 to Brinell 444 to 555. Impact strength is about 75 ft.cndot.lb (102 J) for 80,000 lb/in.sup.2 (552 MPa) yield strength material, and 30 ft.cndot.lb (41 J) for the 140,000 lb/in.sup.2 (965 MPa) material.
Although ADI represented a significant step toward finding a satisfactory solution for this problem, the abrasion characteristics (e.g., wear resistance) of the tines produced with ADI were not completely satisfactory.
Recently, the use of primary iron carbides in conjunction with ADI has been suggested as a way to produce ductile or gray iron chill castings with improved wear resistance properties. As previously noted, primary iron carbides are unwanted by-products which are formed during the production of ductile iron. Although the austempering process normally dissolves all or most of the primary iron carbides, it has been suggested that by altering the heating and cooling parameters, it is possible to produce a surface layer on the casting which is rich in primary iron carbides.
U.S. Pat. No. 5,837,069 to Deards et al., which is expressly incorporated herein by reference, discloses that a primary iron carbide layer can be formed on the surface of either a gray iron or an ADI cast component, such as a camshaft, which is subjected to rolling contact stress or to sliding stress. According to Deards et al., the component is first cast in a conventional chill casting process to give an iron casting which has a pearlitic and/or ferritic structure with its surface portions rich in primary iron carbides (approximately 60% by volume). Next, the casting is heated to and maintained at a temperature (i.e., 801.degree. C.) sufficient to ensure that all of the pearlitic and/or ferritic structure was converted to an austenitic structure but not so long that the primary iron carbides were dissolved. Next, the casting is cooled (i.e., from 801.degree. C. to 380.degree. C.) rapidly enough to prevent the austenite from converting back into pearlite and/or ferrite. Finally, the casting is maintained at 380.degree. C. long enough to ensure that substantially all of the austenite was converted to ausferrite.
The Deards et al. process is deficient in that it requires numerous, time-consuming temperature and processing control steps in order to avoid loss of the primary iron carbides. Further, the Deards et al. process only produces a surface layer of primary iron carbides. Although this may be appropriate for components that endure little or no impact stresses (e.g., camshafts), it is not appropriate for components that endure repeated high impact stresses (e.g., tines). Additionally, many iron castings do not employ a chill casting process;
thus, the Deards et al. process would not be appropriate.
Other examples of processes and articles relating to casting technology include U.S. Pat. Nos. 4,028,099 to Cole et al.; U.S. Pat. No. 4,033,766 to Cole et al.; U.S. Pat. No. 4,054,275 to Cole et al.; U.S. Pat. No. 4,164,148 to Laforet; U.S. Pat. No. 4,312,668 to Mannion et al.; U.S. Pat. No. 4,313,758 to Henning et al.; U.S. Pat. No. 4,452,647 to Sailas; U.S. Pat. No. 4,464,198 to Mannion et al.; U.S. Pat. No. 4,511,401 to Mannion et al.; U.S. Pat. No. 4,635,701 to Sare et al.; U.S. Pat. No. 4,666,533 to Kovacs et al.; U.S. Pat. No. 4,737,199 to Kovacs; U.S. Pat. No. 4,877,435 to Haeberle, Jr. et al.; U.S. Pat. No. 4,880,477 to Hayes, et al.; U.S. Pat. No. 4,913,878 to Dawson et al.; U.S. Pat. No. 4,953,612 to Sare et al.; U.S. Pat. No. 5,043,028 to Kovacs et al.; U.S. Pat. No. 5,122,204 to McDonald; U.S. Pat. No. 5,139,579 to Kovacs et al.; 5,246,510 to Kovacs, et al.; U.S. Pat. No. 5,569,395 to Arnoldy; and U.S. Pat. No. 5,611,143 to Graf, all of which are expressly incorporated herein by reference.
Therefore, there exists a need for ductile iron castings comprised of primary iron carbides uniformly dispersed throughout an ausferritic matrix and methods for preparing the same.